The spectrally sensitizing technique is an extremely important and necessary technique for producing light-sensitive materials having high sensitivity and excellent color reproducibility. Various spectrally sensitizing agents have so far been developed, and many techniques with respect to their use such as supersensitization and the manner of their addition have also been developed. Spectrally sensitizing agents absorb even light rays of a longer wavelength region which a silver halide photographic emulsion does not substantially absorb and in turn transfers the absorbed light energy to silver halide. Therefore, the increase of the amount of trapped light caused by the spectrally sensitizing agent serves to enhance photographic sensitivity. Thus, attempts have been made to increase the amount of the spectrally sensitizing agent to be added to a silver halide emulsion as well as to develop spectrally sensitizing agents with a high light-absorbing coefficient. However, as to the amount of the spectrally sensitizing agent added to a silver halide emulsion, there exists an optimal range, which is usually less than the amount necessary for coating the whole surface of silver halide crystals with it. If the spectrally sensitizing agent is added in an amount more than the above-described optimal range, there results serious desensitization (Mees; The Theory of the Photographic Process, pp. 1067-1069 (1942)).
In an effort to increase the amount of spectral sensitizing agent to silver halide, attempts have been made to absorb two spectrally sensitizing agents, which are in a proper electric potential relation with each other, onto silver halide crystals in layer form to thereby increase the amount of trapped light while depressing desensitization which accompanies the increase in the amount of spectrally sensitizing agent added, as described in, for example, Thomas L. Penner & P. B. Gilman, Jr., Phot. Sci. Eng., 20 (3), 97-106 (1976). However, this technique is ineffective for high performance silver halide emulsions with enough high sensitivity to be used in photographic materials and is far from serving to provide actual photographic light-sensitive materials.
Attempts have also been made to cover silver halide crystals with a spectrally sensitizing agent at a coverage within the optimal region in which no desensitization takes place and yet to increase the whole amount of added spectrally sensitizing agent within the optimal region to thereby increase the amount of trapped light and improve spectral sensitivity. An example of this technique is to use tabular silver halide grains having a large specific surface area as described in Japanese Patent Application (OPI) No. 113926/83, etc. In this technique, however, the optimal coverage of the spectrally sensitizing agent in spectral sensitization tends to be considerably lower than that of other silver halide grains such as cubic grains, regular octahedral grains, tetradecahedral grains, twin grains, etc., and hence the amount of spectrally sensitizing agent cannot be increased much. If the amount of a spectrally sensitizing agent is increased, a reduction in sensitivity results, and high spectral sensitivity will not be obtained. Thus, the aforesaid effects of this technique are not necessarily obtained.
Since tabular silver halide emulsions have a low light absorption coefficient and an extremely low sensitivity in the silver halide-intrinsic absorption region due to their small grain volume, high sensitivity is obtained only when the spectrally sensitizing ratio is much higher than that of other forms of silver halide grains. In view of this, the aforesaid effects cannot be great advantages. However, if high sensitivity is obtained by attaining a high spectrally sensitizing ratio, tabular silver halide grains can produce improvement in the sharpness of the image when used in green-sensitive or red-sensitive emulsions of color light-sensitive materials. This is because tabular silver halide grains can allow the elimination of, or a decrease in the thickness of, a yellow filter layer used for lowering blue sensitivity as the tabular silver halide grains have low blue sensitivity which is essentially unnecessary. In many cases, this yellow filter layer is formed by using colloidal silver, and this colloidal silver can diffuse into contiguous emulsion layers to cause fog. This problem is concurrently eliminated by the above-described tabular silver halide emulsion. The tabular silver halide emulsion may also be used as a blue-sensitive emulsion by using an agent which spectrally sensitizes a blue region as described in Japanese Patent Application (OPI) No. 113926/83. However, application of spectrally sensitizing agents to tabular grain emulsions as suggested in Japanese Patent Application (OPI) No. 113926/83 cannot be called a special technique as has been asserted, and is not different from that applied to other ordinary silver halide grains at all.